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Forensic Chemistry > Lecture 8: Combustion > Flashcards

Lecture 8: Combustion Flashcards

1
Q

What is the combustion reaction in words

A

Fuel + Oxygen —–> Gases + Heat + Light

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2
Q

What is the chemical combustion reaction

A

CH4 + 2O2 ——> 2H20 + Heat + Light

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3
Q

What are the 3 sources of fuel for combustion

A
  • Solid (woods)
  • Liquids (accelerants)
  • Gas (propane, butane etc.)
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4
Q

What does heat do in a combustion reaction

A

Breaks the bonds of the reactants

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5
Q

Why is combustion an exothermic reaction

A

Releases energy

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6
Q

Why is combustion a continuous reaction

A

Combustion will continue until the fuel or oxygen runs out

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7
Q

What are the factors affecting the rate of combustion

A
  • Physical state of the fuel (solid, liquid or gas)
  • The temperature of the flammable liquids
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8
Q
  • What are the different rates of a combustion reaction
A
  • Detonation
  • Deflagration
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9
Q
  • What is detonation
A

The speed of combustion is faster than the speed of sound (sonic), Produces a devastating shockwave which destroys everything

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10
Q

*What is deflagration

A

The speed of the combustion reaction is slower than the speed of sound (subsonic), produces a wave of flame front.

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11
Q

What are the combustible solids

A
  • Pyrolyzable (Woods)
  • Non-pyrolyzable (Charcoal, Cigarettes)
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12
Q

What are pyrolyzable solids

A

Can undergo combustion at high temperatures, without the presence of oxygen.

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13
Q

When wood undergoes pyrolysis what does it produce

A

gaseous products

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14
Q

Gaseous products + Oxygen =

A

Flaming fire + heat

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15
Q

How long does combustion of pyrolyzable solids last

A

Until all solids within the wood have been consumed

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16
Q

What remains following the combustion of pyrolyzable solids

A

a carbon based residue which undergoes glowing combustion, smoldering

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17
Q

What are non pyrolyzable solids

A

Undergoes glowing combustion

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18
Q

What is charcoal

A

Pure carbon

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19
Q

What happens when carbon is heated

A

Doesn’t break down or produce vapours, instead the surface oxidises and undergoes glowing combustion

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20
Q

How does the oxidation of carbon occur

A

Oxidation starts at the outer surface, consumes it and moves onto the next surface and continues until there is nothing left.

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21
Q

What are the constituents of inorganic gun shot residue?

A

-Lead
-Antimony
-Barium

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22
Q

What is required for definitive identification of gun shot residue?

A

Identification of all 3 elements
*As they can be found individually in legal things

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23
Q

What technique is used to analyse inorganic gunshot residue?

A

SEM-EDX

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24
Q

What does SEM-EDX stand for?

A

Scanning electron microscopy - energy dispersive X-Rays

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25
Q

How does SEM-EDX work?

A

-The microscope produces the image of the particle
-X-ray determines the elemental composition

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26
Q

What happens when a bullet is ejected from a gun barrel?

A

-Firing pin strikes the primer, detonating it producing flames
-The flames ignite the propellant which produces gas causing the bullet to travel down the barrel of the gun

27
Q

What are the combustion reactions causing the ejection of a bullet from a gun?

A
  1. Firing pin strikes the primer, detonating it
  2. Fire ignites the propellant
28
Q

What are the series of combustion reaction causing the ejection of a bullet from a gun called?

A

A low explosive train

29
Q

What are the changes in energy causing the ejection of a bullet from a gun?

A

1.Fire pin strikes primer: Mechanical energy
2.Detonation of primer produces heat: Mechanical to heat energy
3.Heat causes the production of gas: Heat to chemical energy
4.Movement of bullet: Chemical to kinetic energy

30
Q

What are the 3 factors affecting the acceleration of a bullet?

A

-Rate of gas production
-Burn rate
-Burn pattern

31
Q

How does the rate of gas production affect the acceleration of a bullet?

A

Steady state production of gas is required to ensure smooth movement of the bullet down the barrel

32
Q

What will happen to the acceleration of the bullet if the rate of gas production is too fast?

A

The initial momentum won’t be maintained and the bullet will slow down as it exits the gun

33
Q

What will happen to the acceleration of the bullet if the rate of gas production is too slow?

A

The bullet will have exited the barrel before all gases have been produced

34
Q

What is a progressive burn pattern?

A

burns from end to the other

35
Q

What is a degressive burn pattern?

A

Burns from the outside in

36
Q

What is a neutral burn pattern

A

Burns in a neutral pattern
produces pores, increasing surface area

37
Q

Why must burn rate be controlled for the acceleration of a bullet?

A

To ensure maximum kinetic energy is imparted to the bullet

38
Q

What shape of propellant particle does a shotgun require?

A

Disc shape

39
Q

What shape of propellant particle does a revolver require?

A

tubular shape

40
Q

What can propellant particles be coated with that inhibits combustion?

A

A detterant

41
Q

What does the detonation speed of an explosive depend on?

A

The density of the explosive

42
Q

How does the density of an explosive impact the detonation speed?

A

Greater density = Faster detonation

43
Q

What are the shockwaves produced by a bullet?

A
  1. Mach cone
  2. Canneluer
  3. Wake
44
Q

What is the shockwave produced at the front of a bullet?

A

Mach cone

45
Q

What is the shockwave produced from the middle of the bullet?

A

Canneleur

46
Q

What is the shockwave produced from the base of the cone?

A

Wake

47
Q

What is a molecular explosive?

A

-Detonate easily
-Explosives in their purest form
-Difficult to handle safely
-Highly unstable

48
Q

What is are examples of a molecular explosive?

A

-Nitroglycerin
-Nitrocellulose

49
Q

What are low explosives?

A

-Deflagration explosives
-Won’t detonate in normal conditions
-Will detonate when confined, producing a throwing action
-Decomposed by a flame front moving sub-sonically through the object

50
Q

What are examples of low explosives?

A

-Nitrocellulose
-Gun powder/ black powder

51
Q

What are the uses of low explosives?

A

-Gun powders
-Flares
-Propellants

52
Q

What are high explosives?

A

-Detonating explosives
-Decomposed by high pressure shock waves which move sonically

53
Q

What are primary explosives?

A

-A high explosive
-Sensitive to shock and impact
-Detonated by shock, impact, friction etc.
-Produces a powerful shock wave that will detonate 2nd explosives and propellants

54
Q

What are examples of primary explosives?

A

-Lead azide
-Lead styphnate
-Mercury fulminate

55
Q

What are uses of primary explosives?

A

-Blasting caps
-Military fuses
-Primers in cartridges

56
Q

What are secondary explosives?

A

-Main explosives
-Will burn under normal circumstances
-not sensitive to shock or impact
-Only detonates due to primary explosive shock waves

57
Q

What examples of secondary explosives?

A

-TNT
-Dynamite (Nitroglycerine)
-HMX

58
Q

What is the full name for TNT?

A

2,4,6-Trinitrotolune

59
Q

What is the full name for HMX?

A

1,3,5,7-tetranitro,1,3,5,7-tetracycloctane

60
Q

What are propellants?

A

-Used to accelerate a projectile to high speed
-Don’t detonate only ignite

61
Q

What is a single base propellant?

A

Nitrocellulose

62
Q

What is a double base propellant?

A

Nitrocellulose + Nitroglycerin

63
Q

What is a triple base propellant?

A

Nitrocellulose + Nitroglycerin + Nitroguanidine

Forensic Chemistry (7 decks)

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